I'm trying to figure out the differences over the years... Here's what I "belive to be right" so far...52-55: Shorter shaft on the breather valve & thin gears, different body plate. 56-59: Same length on the shaft up to 76, as well as body plate, thicker gears.

And that's pretty much all info I've been able to dig up. Can anyone please fill in the blanks? Breather window sizes & gear thickness?